Copper Occur Relatively Commonly in COPPER Vugs
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are generally anhedral, euhedral crystals and crystal aggregates of copper occur relatively commonly in COPPER vugs. Cu Composition: Arsenic occurs in small amounts in Native copper is easily the most famous of all of Michigan’s native copper deposits. Some of Michigan’s minerals. As an ore mineral, native it occurs as copper arsenides (algodonite, copper is very rare throughout the world, although domeykite), but the bulk of it is present in solid minor occurrences are widespread. The greatest solution with the copper. The arsenic content native copper deposits ever found and exploited varies from a few ten thousandths of a percent to are those of the Keweenaw Peninsula in over half a percent - a thousand-fold variation. Michigan’s Northern Peninsula. Copper mineral- Broderick (1929) has shown that in most mines the ization occurs along a narrow belt over 150 As:Cu ratio increases with depth and, in general, kilometers long, stretching southwestward through the higher the ratio the greater its rate of increase. Keweenaw, Houghton, and Ontonagon Counties (Part I). There, native copper occurs in Alteration: Copper from many of the mines is amygdaloids, conglomerates, and fissures ranging coated by a patina of reddish cuprite or black in size from microscopic grains up to masses 14 tenorite, which, in some instances, may have meters long and weighing 382 metric tons formed after the mines were opened. However, in (Rickard, 1905). Similarly large masses of float some deposits, particularly in higher levels, the copper have been found in glacial drift, particularly in the Northern Peninsula. (Rickard, 1905; Kraus, 1924), though native copper erratics are known in glacial drift in Wisconsin, Iowa, Illinois (Crook, 1929), Indiana, Ohio, Western New York, and Pennsylvania. While these large masses of copper Figure 61: Native copper, wire copper with datolite from the Osceola mine, Calumet, Houghton County, field of view 7 cm, A. E. Seaman Mineral Museum specimen No. JTR 1671, Jeffrey Scovil photograph. oxidation was pre-mining in origin (Butler and Burbank, 1929). Malachite forms similar coatings. Pseudomorphs of cuprite (q.v.) after copper are also known. Habits: The aggregate forms assumed by copper are highly variable, and a number of distinctive types have been recognized: Figure 60: Native copper with cuprite (red) and tenorite 1. Grains, blebs, pellets, and masses. Anhedral (black) patina from the Central mine, Central, Keweenaw to subhedral. County. 4.5 x 12 cm. A. E. Seaman Mineral Museum specimen No. JTR 537, Jeffrey Scovil photograph. 2. Masses. The larger pieces commonly are very irregular with a hackly appearance. 3. Crystals and crystal groups. 4. Networks. Interconnecting irregular veinlets, Crystallography: The crystallography of Michigan sheets, plates, and aggregates. native copper was studied in detail by Dana (1886), 5. Thin sheets. Formed in narrow fissures or who identified the following forms: fracture planes. An unusually fine specimen Cube {001} of sheet copper is in the University of Dodecahedron {011} Michigan collection. It measures 1.5 meters long, 0.3 to 0.6 meters wide, and about 5 mm Octahedron {111} thick and shows parallel striations. Tetrahexahedrons {014},{025}, 6. Filiform or “wire.” {012}, {035} 7. Arborescent. Three-dimensional fernlike Trapezohedrons {113}, {112} groups. Some variants are also described as Hexoctahedrons {51018}, “moss copper.” An unusually fine and large {1611},{2312} specimen in the University of Michigan collection is a reticulated aggregate of distorted, elongate, semi-flattened crystals The most common habit is tetrahexahedral, with over a meter long, 15 to 30 cm wide, and 5 {014} and {025} the most common forms. These to8 cm thick. may be combined with the cube, dodecahedron, or 8. Leaf. Dendritic aggregates flattened in one (rarely) octahedron. The cube alone is also plane. relatively common, the dodecahedron less common, and the octahedral form alone much less 9. Brick. Massive, replacing sandstone. common. Wilson and Dyl (1992) showed that 10. Shell. Molds of boulders and cobbles. Also crystal habit may be related to crystal size. In a called “skull copper.” An outstanding preliminary study “of copper crystals 4 mm or less specimen is shown by Kemp (1980, page in diameter, in close association with prehnite and 260). It measures 17.5 x 11.5 x 14 cm, and is commonly also with quartz and calcite..nearly 70% from the Calumet and Hecla mine, Houghton of the copper crystals with identifiable forms County. proved to be the tetrahexahedron {057}, and half 11. Shot. Vesicle fillings. the remainder are this form modified to some 12. Spike. Elongated vesicle fillings. An extent by the dodecahedron.” Larger crystals were extraordinary spike is in the University of found to have a higher proportion of other Michigan collection. Set in an altered basalt tetrahexahedron forms, along with the cube, base, it is approximately a meter long and 5 to octahedron, and dodecahedron. Figures 60 to 66 15 cm in diameter, tapering upward, with a illustrate some of these forms. “flopped-over” point. 13. Pseudomorphs. Replacements of feldspar, calcite, or boulders. 14. Half-breeds. Intergrown masses of copper and silver. 15. Barrel. “Ponderous hackly masses” (Rominger, 1895). Figure 62: Native copper, a rare, 2 cm, hexoctahedral copper crystal from the Osceola mine, Calumet, Houghton County, A. E. Seaman Mineral Museum specimen No. JTR 1672, Jeffrey Scovil photograph. Distortions are widespread, giving rise to crystals with marked pseudosymmetry. Preferential elongation along a single crystal axis yields pseudotetragonal crystals. Examples of rhombohedral pseudosymmetry are also common. Some crystals may be skeletal, cavernous, or hollow. In some, the edges of the crystals project above deeply depressed faces. Also common are striations, pits, growth projections, and wavy surfaces. Twinning on {111} is very common, giving rise to simple contact twins, a few penetration types, and less commonly repeated twinning (Dana, 1886). X-ray powder diffraction data for copper (Calumet and Hecla mine) and for arsenian copper (Houghton County) are given by Berry and Thompson (1962, page 12). Baraga County: Huron Islands: Dana (1892) Figure 64: Native copper, dendritic growth of cubic reports “native copper in granite.” Unconfirmed. crystals from the Calumet and Hecla mine, Calumet, Dickinson County: 1. Cyclops mine at Houghton County, 5.5 x 8.5 cm, A. E. Seaman Norway: Float copper in drift above the iron ore Mineral Museum specimen No. JTR 674, Jeffrey Scovil body. 2. Reportedly the largest mass of float photograph. copper found in Dickinson County was uncovered in the early 1940s during excavation for a filling Figure 63: Native copper, “Fern” copper from the Phoenix mine, Phoenix, Keweenaw County, 4.5 x 5.5 cm, A. E. Seaman Mineral Museum specimen No. JTR Figure 65: Native copper, parallel growth of cubic 1664, Jeffrey Scovil photograph. crystals, Lake Superior copper district, 2.5 x 7.5 cm, A. E. Seaman Mineral Museum specimen No. GBR 569, Jeffrey Scovil photograph. northwest of Franklin mine: A large piece of float copper, showing glacial striations and evidence of partial working by Native Americans, weighing 220 kg and measuring 80x 105x 15 cm is now in the Mineralogical Collection of the University of Michigan (Kraus, 1924). 6. Hancock mine: Very good crystals. 7. Isle Royale mine: In lodes and feeding fissures (Broderick, 1931) and disseminated in epidote. 8. Kearsarge mine: Fine crystals in lodes and fissures, sometimes associated with silver (Broderick, 1931). 9. Osceola mine: Very fine crystals, wire (Number 13 shaft), and sheet copper. Wire copper in an intertwined cluster 10 cm high (specimen JTR 1670) is in the A. E. Seaman Mineral Museum (MacFall, 1983). Vugs with perfect small crystals. The famous sheets occurred in fissures in the diabase on the hangingwall of the Osceola amygdaloid (Rominger, 1895). 10. Quincy mine: Fine crystals, many with Figure 66: Native copper, twinned tetrahexahedrons from dodecahedral habit, and microcrystals in the Central mine, Central, Keweenaw County, 6 x 8 cm, amygdules. 11. Tamarack mine: Sheet copper, A. E. Seaman Mineral Museum specimen No. DCG shell copper (boulder casings), and pseudomorphs 1110, Jeffrey Scovil photograph. after calcite. 12. Winona mine: Fine crystals. 13. Wolverine mine: Excellent crystals, reticulated station in Iron Mountain. It is estimated to have masses, and shot copper. 14. Hancock: Ancient been about a meter across, 1 - 2 cm thick, and mine pits were uncovered in the south parts of “only slightly oxidized” (B. J. Westman, written sections 25, 26, and 27, T55N, R34W (Hinsdale, communication, 1983). 1931). 15. Champion mine: Arborescent crystal Gratiot County: Near Ithaca, T10N, R2W in groups. 16. Six Mile Hill, east slope; southwest of Michigan Basin Deep Drill Hole in the altered Houghton: Copper, calcite, and epidote in a upper basaltic-gabbroic unit. An accessory with network of veins. Copper in masses of 1 to 40 bornite and digenite in albite-chlorite-calcite- kilograms. Seams also contained considerable epidote rock (McCallister et al., 1978). chalcocite and some prehnite and datolite (Rominger, 1895). 17. Pewabic mine: As well- Houghton County: 1. Atlantic mine: Large formed crystals and pseudomorphs of copper after masses. A large calcite vein transected the laumontite and after quartz (Pumpelly, 1873). 18. amygdaloid at which junction it contained nests Other mines that have yielded fine specimens and pockets of copper